The present invention is directed to a method for purifying silicon carbide structures useful in high temperature silicon wafer manufacturing processes. More particularly, the present invention is directed to a method for reducing the iron content of a silicon carbide structure suitable for use in a high temperature silicon wafer manufacturing process. The method results in silicon carbide structures having a substantially iron-free denuded zone at their surface that will not diffuse a problematic amount of iron into the silicon wafer atmosphere during high temperature silicon wafer processing steps.
Silicon carbide structures or parts are generally used inside of a furnace during the high temperature processing of silicon wafers. These parts, commonly referred to as boats, have intimate contact with the silicon wafers as they actually hold the wafers in place during numerous processing steps. Silicon carbide substrates are advantageous for this application for several reasons, including: (1) they can be used for very long periods of time at temperatures exceeding 1150° C. without loosing any dimensional tolerances; (2) they are highly chemically inert; and (3) they are extremely strong. Because of these characteristics, silicon carbide is the preferred substrate material for all high temperature silicon wafer thermal processing.
Silicon carbide substrates generally contain a relatively high level of impurities, such as iron, because they tend to be highly porous, which results in more surface area that can trap impurities and make them hard to remove. Because of this, many silicon carbide structures are coated with a very high purity silicon carbide coating prior to use. This silicon carbide coating is much more dense than the substrate, which reduces the porosity of the surface. The purity of the silicon carbide coating introduced onto the silicon carbide structure, however, can present a major problem at the high temperatures required for some silicon wafer thermal treatment processes. For example, when silicon wafers are processed at temperatures equal to or greater than about 1200° C. in an argon/hydrogen or oxygen/nitrogen atmosphere, iron present in the surface layers or bulk of the silicon carbide coating can diffuse from the silicon carbide coating and into the silicon wafers, resulting in unacceptably high levels of iron contamination in the silicon wafers. The bulk iron content of a high purity silicon carbide coating introduced onto a substrate by chemical vapor deposition is typically about 0.1 to about 10×1015 atoms/cm3. Iron concentrations of 1013 iron atoms/cm3 have been commonly observed in silicon wafers processed in commercially available silicon carbide coated boats. This level of contamination is about 1000 times higher than most silicon wafer users will accept.
One solution to the contamination problem would be for the silicon carbide coated boat manufacturers to deposit by chemical vapor deposition a silicon carbide coating onto the substrate that is at least about 1000 times more pure with respect to iron than the currently utilized coating. Although this would potentially solve the iron-contamination problem, such hyper-pure silicon carbide coatings, if commercially available, are extremely expensive and easily contaminated by the machining processes required after deposition. Further, the analytical methods to directly verify such ultra-high purity are not available.
One solution practiced in the industry to date to reduce the iron contamination in silicon carbide boats has been to grow a relatively thick silicon dioxide layer on the surface of the silicon carbide, which is typically done in the presence of a chlorine containing gas such as hydrogen chloride (HCl) or dichloroethylene alone or in combination with oxygen. This oxide layer acts as an iron diffusion barrier and keeps the iron in the silicon carbide below the oxide layer from outdiffusing and reaching the silicon wafer resting on top of the grown oxide layer. There are two main problems with this approach. First, some of the iron in the silicon carbide layer that is oxidized is captured at the top surface of the oxide layer and contaminates the silicon wafer through outdiffusion during processing. Second, the oxide layer is etched away by some processing ambients, such as hydrogen, and leaves the silicon wafers directly exposed to the contaminated silicon carbide. Re-oxidizing the silicon carbide layer intermittently results in the loss of valuable processing time and increased overall cost. Also, this re-oxidation again traps iron at the surface, which can be released during processing as noted above.
An alternative approach to reducing contamination in silicon carbide structures includes etching at least about 0.1 micrometers but no more than about 10 micrometers of silicon carbide from the surface using a gaseous chlorine trifluoride (ClF3) etching process. This process is typically carried out at a temperature of from about 20° C. to about 600° C. Although this process does remove iron contamination found in the surface layers of the silicon carbide coating, it is generally not capable of removing iron contamination from the bulk of the structure or any silicon carbide coating. This bulk iron can diffuse to the surface of the structure or coating during wafer processing and into the environment surrounding the wafers processed.
Another approach to reducing contamination originating from silicon carbide structures and coatings includes utilizing extensive in-situ cleaning processes to reduce the amount of iron contamination present in the silicon carbide. Typical cleaning processes include at least two steps. The first step is an oxidation of the silicon carbide coating in a furnace in the presence of hydrogen chloride gas or dichloroethylene (with oxygen) at a temperature greater than about 950° C. This step results in the formation of iron chloride in the silicon carbide, which is volatile species that can vaporize from the silicon carbide under certain conditions.
The second step in the in-situ cleaning process involves a series of from about 10 to about 20 furnace processing cycles at a normal silicon wafer processing temperature of from about 1200° C. to about 1350° C. with fresh, low iron silicon wafers included in each run. The atmosphere is typically hydrogen, a mixture of hydrogen and argon, or argon only. The fresh low iron silicon wafers are used to extract the iron from the silicon carbide layer during the processing steps. The progress of boat purification is monitored indirectly by measuring the iron content of the silicon wafers exposed to the silicon carbide coated boat with surface photovoltage. Generally, the boat purification and qualification is complete when the iron content in the thermally processed silicon wafers is less than about 1010 iron atoms/cm3 as measured by surface photovoltage.
Although this in-situ cleaning process does produce high quality silicon carbide boats that will not cause significant iron contamination during high temperature silicon wafer processing, it is very time consuming and expensive. In some cases, more than $100,000 worth of fresh silicon wafers must be sacrificed to produce a purified boat. Also, such purification may require 5 or more furnace runs, which can also significantly increase resulting costs.
As such, a need exists in the industry for methods to purify silicon carbide structures suitable for use in high temperature silicon wafer manufacturing processes in a less costly, time efficient manner. Also, it would be advantageous if the method did not require the sacrifice of a significant number of first quality silicon wafers.